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Diffstat (limited to 'txr.1')
| -rw-r--r-- | txr.1 | 164 |
1 files changed, 164 insertions, 0 deletions
@@ -15365,6 +15365,170 @@ allowing the .code return operator to be used to terminate at any point. +.coNP Macros @ doloop and @ doloop* +.synb +.mets ({doloop | doloop*} +.mets \ \ ({ sym | >> ( sym >> [ init-form <> [ step-form ])}*) +.mets \ \ >> ([ test-form << result-form *]) +.mets \ \ << body-form *) +.syne +.desc +The +.code doloop +and +.code doloop* +macros provide an iteration construct inspired by the ANSI Common Lisp +.code do +and +.code do* +macros. + +Each +.meta sym +element in the form must be a symbol suitable for use as a variable name. + +The +.code doloop +macro binds each +.meta sym +to the value produced by evaluating the adjacent +.metn init-form . +Then, in the environment in which these variables now exist, +.meta test-form +is evaluated. If that form yields +.codn nil , +then the loop terminates. The +.metn result-form -s +are evaluated, and the value of the last one is returned. + +If +.metn result-form -s +are absent, then +.code nil +is returned. + +If +.meta test-form +is also absent, then the loop terminates and returns +.codn nil . + +If +.meta test-form +produces a true value, then +.metn result-form -s +are not evaluated. Instead, the +.metn body-form -s +are evaluated. Next, the loop variables are updated by assigning +to each +.meta sym +the value of +.metn step-form . + +The following defaulting behaviors apply in regard to the variable +syntax. For each +.meta sym +which has an associated +.meta init-form +but no +.metn step-form , +the +.meta init-form +is duplicated and taken as the +.metn step-form . +Thus a variable specification like +.code "(x y)" +is equivalent to +.codn "(x y y)" . +If both forms are omitted, then the +.meta init-form +is taken to be +.codn nil , +and the +.meta step-form +is taken to be +.metn sym . +This means that the variable form +.code "(x)" +is equivalent to +.code "(x nil x)" +which has the effect that +.code x +retains its current value when the next loop iteration begins. +Lastly, the +.meta sym +variant is equivalent to +.cblk +.meti <> ( sym ) +.cble +so that +.code x +is also equivalent to +.codn "(x nil x)" . + +The differences between +.code doloop +and +.code doloop* +are: +.code doloop +binds the variables in parallel, similarly to +.codn let , +whereas +.code doloop* +binds sequentially, like +.codn let* ; +moreover, +.code doloop +performs the +.meta step-form +assignments in parallel as if using a single +.cblk +.meti (pset < sym0 < step-form-0 < sym1 < step-form-1 ...) +.cble +form, whereas +.code doloop* +performs the assignment sequentially as if using +.code set +rather than +.codn pset . + +The +.code doloop +and +.code doloop* +macros establish an anonymous +.codn block , +allowing early return from the loop, with a value, via the +.code return +operator. + +.TP* "Dialect Note:" +These macros are substantially different from the ANSI Common Lisp +.code do +and +.code do* +macros. Firstly, the termination logic is inverted; effectively they +implement "while" loops, whereas their ANSI CL counterparts implement +"until" loops. Secondly, in the ANSI CL macros, the defaulting of +the missing +.meta step-form +is different. Variables with no +.meta step-form +are not updated. In particular, this means that the form +.code "(x y)" +is not equivalent to +.codn "(x y y)" ; +the ANSI CL macros do not feature the automatic replication of +.meta init-form +into the +.meta step-form +position. +Lastly, +.code doloop +and +.code doloop* +do not implement an implicit +.codn tagbody . .coNP Operators @ block and @ block* .synb |